Ice crushing against stationary structures can be dramatic. On May 12, 1986 the north and north-east faces of the Molikpaq caisson facility, during operations at the Amauligak I-65 site in the Canadian Beaufort Sea, encountered an ice floe approximately 7 km×15 km×˜2 m. The ice-structure interaction induced vibration, and throughout a significant part of the 27 minutes the floe was moving, extensive crushing of the ice was observed. Cyclic oscillations of load occurred, reaching 250 MN.
The cyclic oscillation of the structure has been explained in terms of ice spalling. The elastic stress in the ice is partially relieved during each spelling event, where the ice is actually penetrated by the structure. The mechanisms that enable the rapid penetration of ice during a spelling event are complex (Gagnon, 1999). A spalling event generally refers to what happens when a portion of relatively intact ice rapidly separates from the ice contact region and shatters, leading to a sudden drop in load, and a surge of the ice toward the structure during the load drop. The shattered spalls have properties of crushed ice, that is, capable of supporting low pressure whereas the remaining ice, such as the central horizontal region of the ice sheet (known as the hard zone), will remain relatively intact and be capable of supporting high pressure. Following each spelling event the penetration into the ice sheet temporarily ceases and load begins to increase again on the ice in the contact zone as the bulk ice sheet continues to move against the structure and generate elastic stress until the next spalling event occurs. This leads to a characteristic sawtooth load pattern.
The important point is that the structure may experience hazardous oscillations due to ice-structure interaction when the spalling rate is at or less than the resonant frequency of the structure-ice system. Large scale structures, such as the Molikpaq caisson facility, are able to withstand considerable forces, however the vibrations caused by ice-structure interactions are dangerous for personnel and equipment, and may result in a risk against the structural integrity of the facility.
There are several prior art techniques for cutting ice. For example, U.S. Pat. No. 3,521,592 to Rosner et al. teaches a cutter mounted to a prow of a marine vessel with a plurality of rotary vertically extending ice engaging units, each unit presenting an array of radially extending ice chopping blades or cutters. The ice engaging units are desirably movable vertically for positioning for optimum efficiency. FIG. 2 of Rosner et al. schematically shows a unit with a dozen blades or cutters.
There is a need for an efficient mechanism for improving protection of structures during ice-structure interactions.